The present invention relates to a pressure reducer for hydraulic brake systems comprising a stepped piston, which may be loaded by a prestressing force, displaceable within a housing bore, the smaller surface of the stepped piston facing an inlet chamber and its larger surface facing an outlet chamber. The stepped piston includes a sealing ring in a circumferential groove, the sealing ring abutting the inner surface of the housing bore by means of a sealing section of maximum diameter. The pressure reducer also includes at least one connecting channel, in particular an axial groove, extending from the outlet chamber toward the inlet chamber over some part of the length of the housing bore, with the end of the connecting channel provided in the inner wall of the housing bore being sealable by the sealing section.
In a known pressure reducer of the above type (Swedish Pat. No. 303,685), the sealing ring is an O-ring arranged in a circumferential groove of rectangular cross-section. However, it has proved that the sealing section will change its position in operation and, thus, the change-over point at which the outlet chamber is separated from the inlet chamber will change.
In a copending U.S. application of H. Seip, Ser. No. 157,579, filed June 9, 1980, now abandoned, assigned to the same assignee as the present application, a solution to the sealing section changing the change-over point has been presented. In this copending application the sealing ring is formed by a gasket which, by means of its inside circumference and its two front faces, sealingly abuts the corresponding walls of the circumferential groove of the stepped piston. The sealing section is situated near the inlet chamber end of the stepped piston and is formed as a lip. Due to the firm coupling of piston and sealing ring there results an exact definition of the change-over point. If, during operation, there are high pressure differences between inlet and outlet pressures it may, however, happen that the inlet pressure which acts radially outwards on the inner side of the lip will partially press the material of the sealing lip into the connecting channel, in particular into an axial groove. This deformation may obstruct the return of the piston and will lead to increased wear.